This invention relates to improved control systems for interconnecting data terminals. It is known to interconnect a number of data terminals, such as point of sale terminals in a store, in a serial fashion with a contral computer or controller so that data flows in a predetermined order from data terminal to adjacent data terminal and eventually to the central computer. Such systems are shown in U.S. Pat. Nos. 4,035,770 to Sarle and 3,519,935 to Hochgraf for example. The data terminals in such systems have heretofore been connected in a number of loops (usually three). Each loop is connected directly to the central controller and has a plurality of terminals (typically 20 to 65) connected in series. The controIler in these prior systems defines the beginning and end of each loop. In addition each loop can contain a number of subloops, with the ends of the subloops disposed near the controller. The subloops are provided to reduce the amount of time the loop system is inoperative. Since the data terminals are connected in series within a loop, without the provision of subloops a single fault would result in the failure of the entire loop. This can be a very expensive failure in the event of loops containing large numbers of terminals.
The system described in the Sarle patent works quite well for its intended purpose, but it could be improved. For example, the particular structure of the Sarle network in some circumstances results in an undesirable amount of wiring since the subloops each begin and end near the controller. In addition, in Sarle there is no express provision for regenerating the signal or data stream as it flows from data terminal to data terminal. This can become a problem: when a number of terminals in the subloop are powered off. The signal can become so attenuated by the time it reached in operative terminal that the information in the signal is lost. Hochgraf does describe repeater or driver circuits between adjacent terminals, which address this problem. But in Hochgraf the solution for a failure in the loop is to cut off all terminals in the loop located downstream from the failure. That can be inefficient when the fault is located far upstream.
Sarle also teaches the automatic identification of the fault and the switching out of any subloops containing a fault, but the switching is done with electro-mechanical relays wnich are not desirable for use with data systems.
The speeds of prior systems have been limited by the fact that the data signals have been bipolar. Switching of these bipolar data signals heretofore has required transformers, which are relatively heavy, expensive and slow, rather than digital devices which are capable of significantly higher data rates.
In addition, because of the loop and subloop structure of the prior systems, flexibility in the placement of additional terminals in the system was less than could be desired without greatly increasing the amount of wiring.